1. Field of the Invention
The present invention relates to a transistor, in particular to a power transistor using a nitride semiconductor.
2. Description of Related Art
In recent years, active research and development have been made on the application of field effect transistors (hereinafter referred to as FETs) using a group III-V nitride compound semiconductor material such as gallium nitride (GaN) to high-frequency and high-power devices. Nitride semiconductor materials such as gallium nitride (GaN), aluminum nitride (AlN) and indium nitride (InN) are capable of producing various kinds of mixed crystal, as well as heterojunction just like conventionally used arsenic semiconductor materials such as gallium arsenide (GaAs). In particular, at the heterojunction interface of the nitride semiconductor material, high concentration carriers are generated by spontaneous and piezoelectric polarization without doping any impurities. Therefore, if the nitride semiconductor material is used, the resulting FET is likely to be a depletion mode (normally-on) FET and it is difficult to obtain an enhancement mode (normally-off) FET.
In the current power electronics market, most of the devices are normally-off and devices using the nitride semiconductor materials are also required to be normally-off.
As an example of the normally-off transistors, Japanese Unexamined Patent Publication No. 2005-244072 proposes a transistor as shown in FIG. 8 including a GaN channel layer 102, an AlGaN barrier layer 104 formed on the channel layer 102 and p-GaN control layer 106 formed on a selected part of the barrier layer 104.
With this structure, piezoelectric polarization generated at the heterointerface between the GaN channel layer 102 and the AlGaN barrier layer 104 is canceled by piezoelectric polarization generated at the heterointerface between the AlGaN barrier layer 104 and the GaN control layer 106. As a result, the concentration of two-dimensional electron gas below the GaN control layer 106 is selectively reduced, thereby achieving the normally-off characteristics.
However, the conventional transistor unfortunately involves problems of deterioration of electrical characteristics, such as reduction in maximum current (Imax) and increase in on-resistance (Ron), which are important electrical parameters of the power transistors.
The control layer 106 is formed by growing a p-AlGaN layer on the barrier layer 104 and dry-etching it using chlorine gas. The p-AlGaN layer formed as the control layer 106 and the barrier layer 104 are both made of AlGaN of the same Al composition ratio, though the doped impurities are different. Therefore, it is impossible to remove the p-AlGaN layer perfectly by etching without etching the barrier layer 104. As a result, the barrier layer 104 is etched (overetching) or the p-AlGaN layer remains on the barrier layer 104 (underetching).
When the overetching occurs, the barrier layer 104 is thinned down to reduce the concentration of the two-dimensional electron gas generated by the polarization, thereby reducing Imax. For the same reason, contact resistance of source and drain electrodes is increased and resistance between gate sources and gate drains are increased to raise Ron.
When the underetching occurs, on the other hand, the source and drain electrodes which are n-type ohmic electrodes are formed on the p-AlGaN layer residue, thereby increasing the contact resistance. Further, when a reverse bias is applied between the gate and the drain or the gate and the source, leak current is generated to pass through the p-AlGaN layer residue.